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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="typeof.other"></a>Other considerations and tips</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="other.html#typeof.natem">Native typeof support and emulation</a></span></dt>
<dt><span class="section"><a href="other.html#typeof.parties">The three participating parties</a></span></dt>
<dt><span class="section"><a href="other.html#typeof.features">Supported features</a></span></dt>
<dt><span class="section"><a href="other.html#typeof.what">What needs to be registered?</a></span></dt>
<dt><span class="section"><a href="other.html#typeof.limi">Limitations</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="typeof.natem"></a>Native typeof support and emulation</h3></div></div></div>
<p>
        Many compilers support typeof already, most noticeable GCC and Metrowerks.
      </p>
<p>
        Igor Chesnokov discovered a method that allows to implement <code class="computeroutput"><span class="identifier">typeof</span></code> on the VC series of compilers. It
        uses a bug in the Microsoft compiler that allows a nested class of base to
        be defined in a class derived from base:
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="identifier">ID</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">typeof_access</span>
<span class="special">{</span>
    <span class="keyword">struct</span> <span class="identifier">id2type</span><span class="special">;</span> <span class="comment">//not defined</span>
<span class="special">};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">ID</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">typeof_register</span> <span class="special">:</span> <span class="identifier">typeof_access</span>
<span class="special">{</span>
    <span class="comment">// define base's nested class here</span>
    <span class="keyword">struct</span> <span class="identifier">typeof_access</span><span class="special">::</span><span class="identifier">id2type</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">type</span><span class="special">;</span>
    <span class="special">};</span>
<span class="special">};</span>

<span class="comment">//Type registration function </span>
<span class="identifier">typeof_register</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">compile</span><span class="special">-</span><span class="identifier">time</span><span class="special">-</span><span class="identifier">constant</span><span class="special">&gt;</span> <span class="identifier">register_type</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;);</span>

<span class="comment">//Actually register type by instantiating typeof_register for the correct type</span>
<span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">register_type</span><span class="special">(</span><span class="identifier">some</span><span class="special">-</span><span class="identifier">type</span><span class="special">));</span>

<span class="comment">//Use the base class to access the type.</span>
<span class="keyword">typedef</span> <span class="identifier">typeof_access</span><span class="special">::</span><span class="identifier">id2type</span><span class="special">::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
</pre>
<p>
        Peder Holt adapted this method to VC7.0, where the nested class is a template
        class that is specialized in the derived class.
      </p>
<p>
        In VC8.0, it seemed that all the bug-featire had been fixed, but Steven Watanabe
        managed to implement a more rigorous version of the VC7.0 fix that enables
        'typeof' to be supported 'natively' here as well.
      </p>
<p>
        For many other compilers neither native <code class="computeroutput"><span class="identifier">typeof</span></code>
        support nor the trick described above is an option. For such compilers the
        emulation method is the only way of implementing <code class="computeroutput"><span class="identifier">typeof</span></code>.
      </p>
<p>
        According to a rough estimate, at the time of this writing the introduction
        of the <code class="computeroutput"><span class="identifier">typeof</span></code>, <code class="computeroutput"><span class="keyword">auto</span></code>, etc., into the C++ standard may not
        happen soon. Even after it's done, some time still has to pass before most
        compilers implement this feature. But even after that, there always are legacy
        compilers to support (for example now, in 2005, many people are still using
        VC6, long after VC7.x, and even VC8.0 beta became available).
      </p>
<p>
        Considering extreme usefulness of the feature right now, it seems to make
        sense to implement it at the library level.
      </p>
<p>
        The emulation mode seems to be important even if a better option is present
        on some particular compiler. If a library author wants to develop portable
        code using <code class="computeroutput"><span class="identifier">typeof</span></code>, she needs
        to use emulation mode and register her types and templates. Those users who
        have a better option can still take advantage of it, since the registration
        macros are defined as no-op on such compilers, while the users for whom emulation
        is the only option will use it.
      </p>
<p>
        The other consideration applies to the users of VC7.1. Even though the more
        convenient <code class="computeroutput"><span class="identifier">typeof</span></code> trick is
        available, the possibility of upgrade to VC8, where emulation remains the
        only option, should be considered.
      </p>
<p>
        The emulation mode can be forced on the compilers that don't use it by default
        by defining the <code class="computeroutput"><span class="identifier">BOOST_TYPEOF_COMPLIANT</span></code>
        symbol:
      </p>
<pre class="programlisting"><span class="identifier">g</span><span class="special">++</span> <span class="special">-</span><span class="identifier">D</span> <span class="identifier">BOOST_TYPEOF_COMPLIANT</span> <span class="special">-</span><span class="identifier">I</span> <span class="special">\</span><span class="identifier">boost</span><span class="special">\</span><span class="identifier">boost_1_32_0</span> <span class="identifier">main</span><span class="special">.</span><span class="identifier">cpp</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="typeof.parties"></a>The three participating parties</h3></div></div></div>
<p>
        The Lambda example from the Motivation section requires the following registration:
      </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="identifier">BOOST_TYPEOF_INCREMENT_REGISTRATION_GROUP</span><span class="special">()</span>

<span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuples</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">::</span><span class="identifier">lambda_functor</span><span class="special">,</span> <span class="number">1</span><span class="special">);</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">::</span><span class="identifier">lambda_functor_base</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">::</span><span class="identifier">relational_action</span><span class="special">,</span> <span class="number">1</span><span class="special">);</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">::</span><span class="identifier">logical_action</span><span class="special">,</span> <span class="number">1</span><span class="special">);</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">::</span><span class="identifier">other_action</span><span class="special">,</span> <span class="number">1</span><span class="special">);</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TYPE</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">::</span><span class="identifier">greater_action</span><span class="special">);</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TYPE</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">::</span><span class="identifier">less_action</span><span class="special">);</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TYPE</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">::</span><span class="identifier">and_action</span><span class="special">);</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span><span class="special">::</span><span class="identifier">placeholder</span><span class="special">,</span> <span class="special">(</span><span class="keyword">int</span><span class="special">));</span>
</pre>
<p>
        It may seem that the price for the ability to discover the expression's type
        is too high: rather large amount of registration is required. However note
        that all of the above registration is done only once, and after that, any
        combination of the registered types and templates would be handled. Moreover,
        this registration is typically done not by the end-user, but rather by a
        layer on top of some library (in this example -- Boost.Lambda).
      </p>
<p>
        When thinking about this, it's helpful to consider three parties: the typeof
        facility, the library (probably built on expression templates principle),
        and the end-user. The typeof facility is responsible for registering fundamental
        types. The library can register its own types and templates.
      </p>
<p>
        In the best-case scenario, if the expressions always consist of only fundamental
        types and library-defined types and templates, a library author can achieve
        the impression that the <code class="computeroutput"><span class="identifier">typeof</span></code>
        is natively supported for her library. On the other hand, the more often
        expressions contain user-defined types, the more responsibility is put on
        the end-user, and therefore the less attractive this approach becomes.
      </p>
<p>
        Thus, the ratio of user-defined types in the expressions should be the main
        factor to consider when deciding whether or not to apply the typeof facility.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="typeof.features"></a>Supported features</h3></div></div></div>
<p>
        The Typeof library pre-registers fundamental types. For these types, and
        for any other types/templates registered by the user library or end-user,
        any combination of the following is supported:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Pointers;
          </li>
<li class="listitem">
            References (except top-level);
          </li>
<li class="listitem">
            Consts (except top-level);
          </li>
<li class="listitem">
            Volatiles (except top-level);
          </li>
<li class="listitem">
            Arrays;
          </li>
<li class="listitem">
            Functions, function pointers, and references;
          </li>
<li class="listitem">
            Pointers to member functions;
          </li>
<li class="listitem">
            Pointers to data members.
          </li>
</ul></div>
<p>
        For example the following type:
      </p>
<pre class="programlisting"><span class="keyword">int</span><span class="special">&amp;</span> <span class="special">(*)(</span><span class="keyword">const</span> <span class="keyword">volatile</span> <span class="keyword">char</span><span class="special">*,</span> <span class="keyword">double</span><span class="special">[</span><span class="number">5</span><span class="special">],</span> <span class="keyword">void</span><span class="special">(*)(</span><span class="keyword">short</span><span class="special">))</span>
</pre>
<p>
        is supported right away, and something like:
      </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="special">(</span><span class="identifier">MyClass</span><span class="special">::*)(</span><span class="keyword">int</span> <span class="identifier">MyClass</span><span class="special">::*,</span> <span class="identifier">MyClass</span><span class="special">[</span><span class="number">10</span><span class="special">])</span> <span class="keyword">const</span>
</pre>
<p>
        is supported provided <code class="computeroutput"><span class="identifier">MyClass</span></code>
        is registered.
      </p>
<p>
        The Typeof Library also provides registration files for most STL classes/templates.
        These files are located in the std subdirectory, and named after corresponding
        STL headers. These files are not included by the typeof system and have to
        be explicitly included by the user, as needed:
      </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">functional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="identifier">BOOST_AUTO</span><span class="special">(</span><span class="identifier">fun</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">bind2nd</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">less</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(),</span> <span class="number">21</span><span class="special">));</span> <span class="comment">//create named function object for future use.</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="typeof.what"></a>What needs to be registered?</h3></div></div></div>
<p>
        It is possible to take advantage of the compiler when registering types for
        the Typeof Library. Even though there is currently no direct support for
        typeof in the language, the compiler is aware of what the type of an expression
        is, and gives an error if it encounters an expression that has not been handled
        correctly. In the <code class="computeroutput"><span class="identifier">typeof</span></code>
        context, this error message will contain clues to what types needs to be
        registered with the Typeof Library in order for <code class="computeroutput"><span class="identifier">BOOST_TYPEOF</span></code>
        to work.
      </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span> <span class="special">{};</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span><span class="keyword">bool</span> <span class="identifier">B</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">Y</span> <span class="special">{};</span>

<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">,</span><span class="identifier">Y</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">true</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">a</span><span class="special">;</span>

<span class="identifier">BOOST_AUTO</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span><span class="identifier">b</span><span class="special">);</span>
</pre>
<p>
        We get the following error message from VC7.1
      </p>
<pre class="programlisting">error C2504: 'boost::type_of::'anonymous-namespace'::encode_type_impl&lt;V,Type_Not_Registered_With_Typeof_System&gt;' : base
    class undefined
        with
        [
            V=boost::type_of::'anonymous-namespace'::encode_type_impl&lt;boost::mpl::vector0&lt;boost::mpl::na&gt;,std::pair&lt;X,Y&lt;int,true&gt;&gt;&gt;::V0,
            Type_Not_Registered_With_Typeof_System=X
        ]
</pre>
<p>
        Inspecting this error message, we see that the compiler complains about
        <code class="computeroutput"><span class="identifier">X</span></code>
      </p>
<pre class="programlisting"><span class="identifier">BOOST_TYPEOF_REGISTER_TYPE</span><span class="special">(</span><span class="identifier">X</span><span class="special">);</span> <span class="comment">//register X with the typeof system</span>
</pre>
<p>
        Recompiling, we get a new error message from VC7.1
      </p>
<pre class="programlisting">error C2504: 'boost::type_of::'anonymous-namespace'::encode_type_impl&lt;V,Type_Not_Registered_With_Typeof_System&gt;' : base
    class undefined
        with
        [
            V=boost::type_of::'anonymous-namespace'::encode_type_impl&lt;boost::mpl::vector0&lt;boost::mpl::na&gt;,std::pair&lt;X,Y&lt;int,true&gt;&gt;&gt;::V1,
            Type_Not_Registered_With_Typeof_System=Y&lt;int,true&gt;
        ]
</pre>
<p>
        Inspecting this error message, we see that the compiler complains about
        <code class="computeroutput"><span class="identifier">Y</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">true</span><span class="special">&gt;</span></code>. Since <code class="computeroutput"><span class="identifier">Y</span></code>
        is a template, and contains integral constants, we need to take more care
        when registering:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span><span class="special">(</span><span class="identifier">Y</span><span class="special">,(</span><span class="keyword">typename</span><span class="special">)(</span><span class="keyword">bool</span><span class="special">));</span> <span class="comment">//register template class Y</span>
</pre>
<p>
        It is a good idea to look up the exact definition of <code class="computeroutput"><span class="identifier">Y</span></code>
        when it contains integral constants. For simple template classes containing
        only typenames, you can rely solely on the compiler error.
      </p>
<p>
        The above code now compiles.
      </p>
<p>
        This technique can be used to get an overview of which types needs to be
        registered for a given project in order to support <code class="computeroutput"><span class="identifier">typeof</span></code>.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="typeof.limi"></a>Limitations</h3></div></div></div>
<p>
        Nested template template parameters are not supported, like:
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">Tpl</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">A</span><span class="special">;</span> <span class="comment">// can't register!</span>
</pre>
<p>
        Classes and templates nested inside other templates also can't be registered
        because of the issue of nondeduced context. This limitation is most noticeable
        with regards to standard iterators in Dinkumware STL, which are implemented
        as nested classes. Instead, instantiations can be registered:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_TYPEOF_REGISTER_TYPE</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">const_iterator</span><span class="special">)</span>
</pre>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2004, 2005 Arkadiy Vertleyb, Peder Holt<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">
        http://www.boost.org/LICENSE_1_0.txt </a>)
      </p>
</div></td>
</tr></table>
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